The present invention relates to an LED driver of an LED printer used for electrophotographic image formation.
A variety of means for LED printers have been heretofore proposed for this type of electrophotographic system in order to drive a large number of LED array chips at high speeds and yet provide a sharp photoimage. The following two types are illustrated for the conventional typical LED drive mechanism:
They are, namely, a configuration in which a drive IC and an LED array chip are connected on a 1:1 basis and another configuration which calls for connecting in parallel a plurality of LED array chips to a single drive IC. An example of the first configuration which enables high-speed printing is illustrated in FIG. 2. This configuration calls for arraying, in a linear row, a plurality of LED array chips (A) which are provided with a plurality of LEDs, where each LED array chip (A) is connected to a drive IC (B) on a 1:1 basis so as to simultaneously start the large number of drive ICs and execute a single line block in one shot dot printing. Such configuration features high speed dot printing.
A specific example for the second configuration which permits reducing a run time electric current consumption and thus the operating costs, is illustrated in FIG. 3.
This configuration calls for a parallel connection of a single drive IC (B') to a single row of a plurality of LIED array chips (A') and a connection of each array chip at its common side via a switching element (C') to scanning means (D'), thus permitting each LED array chip (A') to partake m a time-sharing scanning. This configuration requires only one drive IC (B'), which substantially reduces not only the fabrication costs, but also the run time electric current consumption.
The foregoing configurations illustrated by these two conventional examples can be effective for increasing the dot print speed or reducing the electric current consumption, respectively. However, each of these conventional examples is still left with an unsolved problem.
That is, the type illustrated in FIG. 2 requires an individual drive IC (B) for each LED array chip (A) with an inevitable substantial increase in the manufacturing cost. The dot printing of particularly large-size paper presents a problem in that, for example, dot printing AO-size paper needs 14,000 dots, which m turn requires 110 LED array chips (A) because a single LED array chip (A) provides 128 dots, and which also requires as many as 110 corresponding drive ICs (B). This requirement results in higher manufacturing costs. Additionally, the dot printing will turn on all of the many LED array chips (A) for the single array block on a linear row, and thus all of the LEDS simultaneously turned-on would require that the maximum electric current consumption (I.sub.max), at an electric current consumption of 5 mA per bit of LED, be as given by the following equation: EQU I.sub.max =5 mA.times.128.times.112=71.68 A
Thus, such a configuration requires a larger power source and is plagued with a problem of dissipating the heat generated.
The parallel configuration illustrated in FIG. 3, which calls for a time-sharing scanning of a plurality of LED array chips (A') can solve the problems of the configuration illustrated in FIG. 2, but still suffers from the extended time required for completing the dot printing of an array block, and will resist any improvement in printing efficiency. Such configuration is not be expected to provide uniform and sharp dot printing when used for printing large-size paper.
That is, for example, printing large-size paper, such as an AO size, under conditions of: (1) 200 microseconds for turning on the LED per dot; (2) the number of required LED array chips being 110; (3) the time (T) required for a single scanning being 22 milliseconds, will mean that (4) if the dram rotates at a speed of 40 mm/sec, then the line dot width (W) from the preceding line to the next line is 0.88 mm. If the resolution of a printer is, for example, 400 DPI, the line dot width must be 0.064 mm, so that the above printing speed for this configuration cannot hope to provide sharp printing. This will force the reduction of the drum speed for improved print quality.
A lower printing speed problem due to a low dram speed may be circumvented by a shorter turn-on time per dot, but then the insufficient LED emission will result in poor print quality. Another approach may involve, for example, using an LED with higher lumincscence, which would be cost prohibitive, making neither approach practical.